This invention relates to apparatus for controlling a rapid feed speed of a movable member, for example, a table, saddle or the like of a machine tool.
Although a ball screw has been widely used for moving a movable body of a machine tool and various machines, in recent years, a high speed operation of a machine has been developed for the purpose of increasing productivity. Usually, a ball screw is constructed such that it is supported on a machine bed or the like by supporting both ends of the ball screw through bearings, that one end of the ball screw is connected to a source of drive, and that a nut mating with an intermediate portion a threaded rod of the ball screw is connected to the movable body so as to transform the rotation of the source of drive into a linear motion of the movable body. As a consequence where the distance of movement of the movable body, that is, the table is large, for example larger than 6 m, the maximum number of revolutions is limited by a dangerous speed resonating with the natural or inherent number of vibrations of the ball screw. For this reason, for the purpose of preventing deflection of the ball screw, according to a prior art construction, a bracket is provided at an intermediate position of the ball screw, which is retracted from the ball screw when the nut approaches the bracket and advanced toward the ball screw when the nut has passed through the bracket, thus pressing the bracket against the ball screw for preventing its deflection. However, this construction is complicated and friction is created between the bracket and the periphery of the ball screw. Moreover, where the retraction motion is not adequate or a misoperation occurs, the machine would be damaged.
To eliminate these problems, as has been disclosed in Japanese Laid Open Utility Model Specification No. 27352/1984 a deflection preventing device has been proposed wherein a pair of brackets interconnected by a rod are slidably mounted on a bed so as to clamp the nut of the ball screw. Although this construction is effective it is necessary to provide two brackets, rods interconnecting them and guide planes for guiding the brackets.